Fitting components and unions are commonly used to sealingly connect a tube to another device, to another tube, or simply to cap the tube. When used in analytical systems, fitting components and unions are most often used to sealingly connect two tubes together, in order to allow leak-tight fluid communication between the tubes. Fitting components can also be part of analytical devices and actuating mechanism for receiving different types of tubing.
One common type of fitting assembly 10 is shown in FIGS. 1 and 1A (PRIOR ART). A double ferrule 12, formed by a front ferrule 12a and a back ferrule 12b, pinches a tube 14 near its extremity, creating a bulge frontward of the ferrule 12, commonly known as a “swaging” of the tube 14. This swaging provides a good grip on the tube 14.
Double ferrule fitting assemblies are largely used in industrial applications such as in high pressure systems and/or in applications in which there is a high level of vibration. The bulging extremity of the tube 14 makes it very difficult to remove the tube 14 from the fitting 16 and thus creates a safe, seal-tight connection.
The widespread use of double ferrule fitting assemblies in industrial applications, along with their widespread availability, has led analytical system designers to use them in analytical instruments and sampling systems. The following paragraphs describe some of the drawbacks of fittings having a “swaging” action.
Scratches Generating Particles and Eventually Causing Leaks
Packed columns in gas chromatographic instruments must often be changed. A common reason for replacing the columns is the need for measuring new types of impurities in a new sample background. The outside diameter (OD) of these columns is typically of either 1/16″ OD or ⅛″ OD, and less frequently of ¼″ OD.
Referring to FIG. 1A, it is the “swaging” action of the tube 14 within the fitting 16 that creates the sealing and the tube gripping. The required torque to achieve proper sealing increases each time the tube 14 is inserted and retrieved from the fitting 16. When increasing the torque, the tubing 14 is forced deeper into the body of the fitting 16, although at some point the tube 14 cannot be moved forward, and its outside diameter cannot become larger, since the tube 14 is surrounded by the body of the fitting. With frequent assembly and disassembly of the tube 14 and the fitting 16, it becomes more and more difficult to pull out the tube 14 from the fitting 16, and even more difficult to re-insert the tube back in the fitting 16. This frequent assembly/disassembly of the tube 14 and the fitting 16 generates scratches inside the fitting 16 which in turn generates particles and eventually creates leaks at these locations.
In order to overcome these problems, one practice consists of cutting the tube just frontward of the front ferrule or of withdrawing the tube a little before tightening the nut 17, in order to eliminate the bulging of the tube1. While this practice reduces the difficulty to remove and reinsert the tubes within the fitting, it eliminates by the same occasion the safety properties, i.e. tolerance to very high pressure and vibration, of the swaging double ferrule type fitting. Even worse, this practice leads to another problem which consists in the creation of larger dead volumes. 1Agilent, 6890 User's Manual and, Site Preparation and Installation Manual
Dead Volume
In trying to resolve the problem caused by the “swaging” of double ferrule fittings, users have created a problem difficult to deal with, which are larger dead volumes. Indeed, by cutting/withdrawing the tubing', a larger volume between the extremity of the tube and the back, or seating portion, of the fitting is created since the space or volume previously occupied by the tubing is left empty.
With reference to FIG. 2, a simple gas chromatography (GC) system 18 is shown. In this case, the dead volume is present on both sides of the column since there is a fitting 10 on each end of the column. These dead volumes become problematic when there is a low carrier flow. Indeed, this will generate chromatographic peak broadening. Problems caused by scratches and generated particles are relatively easy to detect. However, problems caused by dead volumes are much more subtle, and can sometimes be mistakenly identified as leaks. In fact, dead volumes are often referred to as virtual leaks.
Still referring to FIG. 2, a sample gas 19 is injected on a separation column to separate the impurities and then to measure them by the integration of successive signal peaks by the detector 21, as well known in the art. The sample loop is swept by the sample gas 19, while the separation column and the detector 21 are swept by the carrier gas 23. In this example, the carrier 23 is helium, the column has an outer diameter (OD) of ⅛″, a molecular sieve is used and the detector 21 is of the helium ionisation type. Such configuration is commonly used for permanent gas measurements. Each side of the column is provided with a double ferrule fitting 10, similar to the one illustrated in FIG. 1A. After starting up the system 18, helium is circulated and the column is regenerated to purge away any contaminants.
FIGS. 3 and 4 are graphics showing the level of impurities in parts per million (ppm) detected in function of the time in minutes. The graphic of FIG. 3 shows the signal of the detector of the system 18 from FIG. 2, after the system has stabilized, while the graphic of FIG. 4 shows the effect of varying the flow of the carrier on the detecting signal. In this case the variation consists of decreasing the flow of the carrier and then of restoring it. When carrier flow is decreased, the signal increases due to the presence of accumulated gas in the dead volumes, this accumulated gas diffusing back into the carrier. The presence of accumulated gas in the carrier increases the impurity level into the detector, thus increasing the detecting signal.
Restoring the flow of the carrier in the system dilutes the impurity level into the carrier gas, causing the signal to decrease. As it can be observed in the graphic of FIG. 4, the signal is lower after the restoration of the flow, in comparison to the signal at the beginning of the trend. This situation can be explained by the fact that there is less contaminant entrapped in the dead volume. Varying a system flow or pressure is a known method for finding leaks in gas chromatography system. However, when analyzing the signal trend of FIG. 4, one could think that there is leak and/or air diffusion in the system. A person skilled in the art would typically retighten the fittings until the signal decreases.
By retightening the fittings, the ferrules are pushed forward in the body of the fitting and the outer diameter of the tubing increases once again, thus decreasing the dead volume. By doing so, the entrapped contaminant is forced back into the carrier gas and detector.
Now referring to the graphic of FIG. 5, the signal shown illustrates the result of this action. Varying the flow or pressure to crosscheck for leaks would again generate a signal similar to the one illustrated in FIG. 4, but with less amplitude. Again, with the best intention in mind, an operator observing this would once again retighten the fittings, believing there are still leaks. The fact that there are also unions and other fittings at various locations in the system makes this problem even more difficult to track, identify and resolve. In the end, in attempting to resolve these virtual leaks, fittings will become over-tightened, and real leaks can be generated.
Single Ferrule Fitting Used in Analytical System
FIGS. 6, 6A and 6B show a single ferrule fitting assembly 20 commonly used in gas chromatography systems. The single ferrule 22 used in such an assembly 20 does not cause a “swaging” action, and the extremity of the tube 24 does not bulge out for holding the tube 24 in place in the body of the fitting 26. When the nut 28 is screwed in the fitting 26, the front edge of the ferrule 22 will grip the tube 24, creating a first sealing area. Another sealing point 33 is obtained between the external surface of the ferrule 22, and the internal surface of the fitting 26. The torque required to screw the nut 28 and push the ferrule 22 frontward in the fitting 26 is generally smaller than the torque required in the double ferrule design. In the double ferrule design, it requires extra torque in order to properly deform the tubing. With single ferrule fittings such as the fitting 26 shown in FIG. 6A, there is normally no deformation of the tube 24. In other words, the portion of the tube extending frontward of the ferrule 22 stays round and straight. The bottom or seating flange of the fitting 26 is where the square end of the tube seats within the fitting.
Best shown in FIG. 6B, the single ferrule fitting minimizes the formation of a dead volume precisely because the deformation of the tube 24 is reduced or eliminated. In order to prevent the tube 24 from being deformed, its diameter must be small enough so that the tube 24 can be slipped and fitted just tightly enough in the inner section of the fitting. Furthermore, the end of the tube 24 must be cut orthogonally, and have a clean and neat finish, in order to create a proper sealing surface with the corresponding squared bottom of the fitting.
Single ferrule fittings generally provide adequate results when the tubing size is smaller than ⅛″ OD. As such, these fittings are sometimes referred to as “zero dead volume” fittings. However, a dead volume is still present in the fitting when in use, even if it is a small one. In particular applications, where high sensitivity systems are used, such as mass spectrometers and plasma emission detectors, the effect of small dead volumes can be observed.
Still referring to FIG. 6B, the dead volume 29a corresponds to the clearance between the outside diameter of the tube 24 and the internal surface of the aperture of the fitting 26. This dead volume 29a, no matter how small, will eventually be filled with fluid. It should be remembered that the diameter of the molecule of Helium is about 0.25 nm, Helium being a carrier commonly used in analytical systems. There is also a larger space or dead volume 29b located between the contact point 33 of the ferrule with the body of the fitting and the location where the tube enters into the pilot zone, ie where the tube 24 extends out of the ferrule 22. When temperature or pressure suddenly changes, these various volumes will eventually be filled with fluid.
In the single ferrule design, similar to the one shown in FIG. 6A, there is no real swaging action. However, in some cases, when tightening the nut to make a tube connection, the rotation force of the nut will be transmitted to the ferrule that also begins to rotate. The front portion of the ferrule will then rotate against the internal surface of the fitting body. This will eventually scratch the surface generating particles and leaks. Furthermore, there is a risk of twisting the front portion of the ferrule relatively to the rear portion. This will make it difficult to reseal the assembly during subsequent manipulations. Another common problem in single-ferrule industrial fittings such as the one illustrated in FIG. 6A is the loosening of the tubing 24 inside the fitting 26.
The Effect of Dead Volumes on Gas Chromatography Systems
Another erratic behaviour caused by dead volumes in chromatography systems can be observed when injecting a relatively large volume of a sample. Indeed, injecting a large volume of a sample suddenly reduces the pressure of the system, generating a “ghost” peak. This “ghost peak” is caused by trapped contaminants in the dead volume, diffusing back into the carrier. The larger is the tubing or the more sensitive is the system, the worse the problem will be.
As it can be seen from FIGS. 7 and 7A, a single ferrule fitting 26′ is shown modified. This set-up allows monitoring the pressure variation in the internal volume of the assembly 20. A capillary hole has been pierced in the fitting 26′ and an external capillary tube 25 is brazed in the body of the fitting 26′. On the other end of the capillary tube 25, a pressure transducer 27 is connected, and a pressure signal is monitored and trended by a data acquisition system 31. FIG. 7A shows more in detail the location where the pressure is measured, at the interface of the fitting 26′ and the tube 24. The ferrule 22 is shown partially.
The graphic of FIG. 8 shows the pressure measured in the fitting assembly 20 of FIG. 7, in function of time. It can be seen that between times T0 and T1, the system is at atmospheric pressure P1. At T1, the system is pressurized at a pressure P2. Slowly the signal of the pressure transducer ramps up until P2 is reached. At T3, the pressure of the system is reduced to P1. Starting at T3, the signal of the transducer decreases until P1 is reached. Thus, between T3 and T4, the fluid entrapped in the dead volume is diffusing or depressurizing back in the main stream, this situation leading to a potential risk of contamination. It should be remembered that in some analytical applications, molecules are counted and that there can be a lot of them in this volume. Reducing this volume would certainly be beneficial.
Problems Related with Torque
It is known that smaller tubes require less torque to achieve a proper sealing. Tubes of 1/16″ OD or 1/32″ OD require less torque from the nut than those of ⅛″ OD or ¼″ OD. Most packed columns are made with ⅛″ OD 304 stainless steel tubing, file cut. With this size of tubing, it is very hard to rotate the nut when it comes in contact with the ferrule. A higher rotating torque is required in order to move forward the nut so that the ferrule grips the tube. This operation is too difficult to perform while simply holding the fitting body in one's hand. Longer tools are required and very often tools such as vise grips are used to hold the fitting. Adding to the difficulty, these fittings must sometimes be replaced on columns located inside gas chromatography ovens or on critical and fragile components of analytical systems.
Another problem comes from the fact that the rotational torque applied on the nut is transferred to the ferrule, which then rotates or twists. Since the contact area between the nut and the ferrule is larger than the contact area between the tip of the ferrule and the body of the fitting, the rotational traction force is large and cannot be counterbalanced by the ferrule. Therefore, until the ferrule becomes really compressed on the tube and forced against the fitting body, it will rotate and/or twist. The rotation of the ferrule on the internal surface of the body of the fitting results in scratches on this surface, and eventually creates leaks. Such a problem is common for tubes having a size of ⅛″ OD and even worse for tubes having a size of ¼″ OD.
FIG. 9A shows how the nut 28 is turned in order to push the ferrule 22 towards the innermost portion of the fitting 26. FIG. 9B shows an undesired rotation of the ferrule 22, due to the transfer of the rotational movement of the nut 28 to the ferrule 22. The rotation of the nut 28 should result in a linear and frontward motion of the ferrule 22 in the fitting. For the reasons explained in the above paragraph, the rotational movement of the nut 28 often results in a rotational movement of the ferrule 22 against the inner surface of the fitting 26.
In light of the above, there is a need for improving the sealing a tube inserted in a fitting component, may it be a valve cap, a union or an actuating mechanism. There is also a need to further reduce dead volumes in fitting components. Yet still, there is a need for reducing or eliminating the rotation of the ferrule inside the fitting component. There is also a need to reduce the torque required to turn the nut in a compression fitting component.